Quantcast Factors Under Hydro-project Control (Cont.)

 
  
 
Water movement (flow) has long been known to stimulate algal growth. Lawrence
Whitford (1960) noted that Ruttner (1926) had first begun to describe this effect. Whitford
quoted Ruttner (1953), "In quiet or in weakly agitated water the organisms are surrounded by a
closely adhering film of liquid, which speedily forms around the animal or plant, a cloak
impoverished of substances important for life. In a rapid current, however, the formation of such
exchange-hindering investitures is prevented, and the absorbing surfaces are continually brought
into contact with new portions of water as yet unutilized." Ruttner continued to note that moving
water "is not absolutely but rather physiologically richer in oxygen and nutrients."
Under some conditions the type of movement and its intensity can produce different
results. The intensity and duration of turbulent mixing affects both the total production as well as
the type of algal dominant. Sufficient turbulence can also disrupt structural integrity of
cyanobacterial filaments and colonies (Thomas and Gibson 1990a & b). Whereas nutrient
limited conditions can be overcome through low-intensity turbulence (Rhee et al. 1981) and thus
promote growth, increased turbulence can inhibit growth (Paerl 1990).
To provide a mitigating effect to water quality, mixing must be of sufficient intensity to
overcome the self-positioning tendency of cyanobacteria that produce gas vesicles (Reynolds
1987). Artificial mixing must continue throughout the favorable growth period for cyanobacteria
if it is to be effective in limiting growth (Smith 1988, Shapiro 1990). Even short quiescent
periods are sufficient to allow rapid re-emergence of cyanobacterial blooms (Paerl 1987). The
decision to address algal growth problems using hydraulic mixing is a complex decision that
requires a firm commitment to the treatment.
Nutrient Limitation: Lakes with existing algal blooms can be enhanced by controlling the
supply of nutrients to the aquatic ecosystem. However, such nutrient control may be less
feasible in certain lakes unless costly measures are taken to accomplish the control. The primary
nutrients of concern are N and P. Control of P has been effected in many aquatic environments.
Although decreasing P loading to lakes has been the common goal of many attempts to
control eutrophication, our understanding of the ecological processes controlling the distribution
and abundance of specific algal types is incomplete. Only the most general statements can be
made and any of them may be violated in specific lakes under specific circumstances.
A lake dominated by a nuisance growth of macrophytes and green algae may respond
favorably to decreased P loading. However, if sediments contain large quantities of P, then they
can potentially become a source of P to the lake rather than the sink they were during sediment
P accumulation. A long history of eutrophication may require a lengthy or expensive time for
recovery from eutrophication.
If nitrogen controls are engaged, then N limitation in the presence of abundant P can
occur. This has the potential for shifting the competitive advantage from eukaryotic algae to
cyanobacteria capable of N-fixation (Smith 1983, Howarth et al. 1988). Cyanobacteria are
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